PURPOSE: To attain execution of plural instruction sets by using a read-only control memory part and a read/write control memory part.

CONSTITUTION: Control signals 10 and 18 are delivered properly with a multiplexer 14 set at the side of a read-only control memory part 1. Thus it is possible to write the information given from outside to a read/write control memory part 11. If a microprogram is incorporated to the part 1 to switch the multiplexer 14 toward the part 11 after reading the information given from outside and writing it to the part 11, a microprocessor works with the information read from outside used as the control memory. Then said information is changed properly to be different is action even through an instruction register 3 has the same contents.